Have you ever looked at a spinning fan where it seemed like you could see the blades, rotating at a speed much lower than they should be? That’s aliasing. Your eye doesn’t sample an image often enough
I don’t think this is right. The effect can be seen in video recordings due to shutter speed, and under some kinds of electric lights which flicker at high frequency. But if you look with your own eyes in daylight, a spinning fan will just look blurry. Human eyes have no shutter and don’t take point samples.
The same goes for sound. With sampling, a signal above the Nyquist frequency will “wrap around” and seem like it’s lower frequency. But with human ears, no. A sound above the cutoff frequency of your ears simply won’t be heard.
Here’s another fun discrepancy between Fourier/Nyquist/etc and what our senses actually do. Imagine a sawtooth wave at 1Hz. As a trigonometric series, it’s a sum of many harmonics. But of course your ears won’t hear that sound as a uniform hum of many notes at once. They’ll hear it as a periodic click once per second.
Due to things like this, it takes a little bit of practical intuition to apply Fourier/Nyquist/etc to signal processing intended for humans.
Huh, that’s weird, because I have in fact experienced what I described many times while looking at fans (physically, not through video)! The blades would seem to be spinning slowly, and sometimes reverse direction. I hadn’t ever even heard of aliasing, it was just mysterious to me.
Was it indoors or outdoors? Like I said, electric lights often have a flicker frequency (50 or 60 Hz depending on country, or some multiple of that).
The same thing can happen with musical instruments btw. When I play guitar indoors, sometimes I can see the string “wobble”, much slower than the sound frequency ought to be. I’m pretty sure that’s because of the interplay between string frequency and light flicker. In sunlight it doesn’t happen.
It was almost always indoors, so that might have been it. I just did an experiment: I turned on a fan I have in my room, while no electric lights were on (only sunlight through the window). Looking at the blades, they indeed seemed like a blur that didn’t resolve into individual blades. However, when I changed the speed of the fan, it seemed like a part of the blur resolved into a bunch of blades that slowed down their spin and changed directions. The blades didn’t fully resolve into visibility, and there were more blurry kind of visible blades than there should have been blades in the fan, but it was very clearly happening (multiple times as I changed the speed back and forth, though not every time I changed the speed). Weird.
I don’t think this is right. The effect can be seen in video recordings due to shutter speed, and under some kinds of electric lights which flicker at high frequency. But if you look with your own eyes in daylight, a spinning fan will just look blurry. Human eyes have no shutter and don’t take point samples.
The same goes for sound. With sampling, a signal above the Nyquist frequency will “wrap around” and seem like it’s lower frequency. But with human ears, no. A sound above the cutoff frequency of your ears simply won’t be heard.
Here’s another fun discrepancy between Fourier/Nyquist/etc and what our senses actually do. Imagine a sawtooth wave at 1Hz. As a trigonometric series, it’s a sum of many harmonics. But of course your ears won’t hear that sound as a uniform hum of many notes at once. They’ll hear it as a periodic click once per second.
Due to things like this, it takes a little bit of practical intuition to apply Fourier/Nyquist/etc to signal processing intended for humans.
Huh, that’s weird, because I have in fact experienced what I described many times while looking at fans (physically, not through video)! The blades would seem to be spinning slowly, and sometimes reverse direction. I hadn’t ever even heard of aliasing, it was just mysterious to me.
Was it indoors or outdoors? Like I said, electric lights often have a flicker frequency (50 or 60 Hz depending on country, or some multiple of that).
The same thing can happen with musical instruments btw. When I play guitar indoors, sometimes I can see the string “wobble”, much slower than the sound frequency ought to be. I’m pretty sure that’s because of the interplay between string frequency and light flicker. In sunlight it doesn’t happen.
It was almost always indoors, so that might have been it. I just did an experiment: I turned on a fan I have in my room, while no electric lights were on (only sunlight through the window). Looking at the blades, they indeed seemed like a blur that didn’t resolve into individual blades. However, when I changed the speed of the fan, it seemed like a part of the blur resolved into a bunch of blades that slowed down their spin and changed directions. The blades didn’t fully resolve into visibility, and there were more blurry kind of visible blades than there should have been blades in the fan, but it was very clearly happening (multiple times as I changed the speed back and forth, though not every time I changed the speed). Weird.
Yeah, I can’t check right now, but my guess is some electrical effect when changing fan speed.